Power over Ethernet (PoE) is a technology that permits the powering of appliances through the same cable that connects them to a LAN. In a PoE network there is essentially a Power Sourcing Equipment (PSE) that is capable of supplying a Powered Device (PD), that may be, for instance, a phone, a webcam, or any other device that is connectable to the LAN. The PSE supplies power to the PD devices connected to respective sockets through a dedicated pair of insulating wires of the connecting cable. In particular, the PSE verifies whether a PD is connected to a LAN line, and monitors the current delivered to the LAN line of the PD continuously. If the PD is disconnected, the PSEs stop the application of the DC supply voltage to the LAN line.
The standard specifications that rule this technique of powering devices contemplate the possibility for the PDs connected to the LAN of communicating to the PSE the power that should be supplied to them for a correct functioning.
In order to verify whether a valid PD is connected or not to an output port of the PSE, the PoE transmission standard that at present is the IEEE 802.3af, contemplates that any connectable PD be provided with a “signature element,” that may be a dedicated circuit with a certain impedance connected between the power supply terminals of the PD. When a new PD is connected to a respective point of the PSE, the PSE carries out an interrogation routine for reading the signature element. At the end of the interrogation routine, if validly identified, the newly connected PD is powered by the PSE.
The standard proposes that the PSE monitor the connection to a PoE supplied PD for verifying whether or not it has been disconnected, and when so, the PSE stops the provision of a supply voltage to the respective line of the LAN, in order to prevent application of a relatively large DC voltage to an open pair of insulated wires of the LAN line.
The procedures for recognizing a valid PD are normally carried out by measuring the resistance seen across the terminals through which the PSE supplies the LAN line, for example by applying a voltage across the pair of power supply conductors of the LAN line and measuring the current absorbed, or by forcing a current and sensing the voltage.
Unfortunately, by using these known recognition procedures, it happens sometimes that a valid PD may be incorrectly recognized as invalid, or vice versa.